There is not a law stating they cannot call you while you are at work. If you owe them a debt, they can call whatever number you provided them with at the time of application, including your job. Although there is not a law that prohibits them from calling you, if you tell them you cannot take personal calls at work the calls should stop.
If you dont want to be called at work dont give them your work number. If you have no other choice but to give your work number, give them your work number than call customer service to have the number removed.